Recently, regarding a lens barrel of an imaging optical system typified as a photographing lens of a camera, there have been increasing ones using a zoom lens in which positions of a plurality of lens groups are changed and the focal length is continuously changed when photographing. In this type of lens barrel, a mainstream type is a so-called collapsed-type zoom lens, which attempts to minimize its size so as to make distances between lens groups narrower and store lenses in a fixed cylinder of the lens barrel when non-photographing. As a structure of moving lenses in the lens barrel of the zoom lens and the like, there has been increasingly used such a mechanism in which a rotating cylinder is held by a fixed barrel, and when the rotating cylinder rotates relative to the fixed cylinder, a lens frame moves forward and backward in an optical axis direction by a cam engagement or a helicoid engagement. As a typical mechanism, there is such a mechanism in which a fixed cylinder and a rotating cylinder are in a cam engagement so as to move the rotating cylinder forward and backward followed by a cam groove while its rotation, inside the rotating cylinder there is a linearly-guiding cylinder which is rotatable relative to the rotating cylinder and is movable integrally with the rotating cylinder in the optical axis direction, and inside the linearly-guiding cylinder there is a lens frame which holds a lens. The lens frame has a cam follower. The cam follower penetrates a linear groove extended in the optical axis direction of the linearly-guiding cylinder, engages with a cam groove provided in an inner surface of the rotating cylinder and thereby when the rotating cylinder rotates, the lens frame, without rotations, is movable to an arbitrary position within a range of a movement by following along a shape of the cam groove.
For example, a rotating key provided protrusively in a radial direction in the linearly-guiding cylinder engages with a rotating groove provided in an inner diameter of the rotating cylinder, and thereby the linearly-guiding cylinder is rotatable around the optical axis relative to the rotating cylinder, also thereby the rotating cylinder and the linearly-guiding cylinder is integrally movable in the optical axis direction. Additionally, the fixed cylinder and the rotating cylinder are in a helicoid engagement during a part of a process from a non-photographing state to a photographing state, and thereby the rotating cylinder is rotatable relative to the fixed cylinder and moves relatively in the optical axis direction. In the photographing state, a flat part of a male helicoid of the rotating cylinder, which is provided in front and rear ends of the male helicoid in the direction of the optical axis and follows along a flat surface where an optical axis crosses perpendicularly, engages with an engaging groove provided in the fixed member, which follows along a flat surface where the optical axis crosses perpendicularly. In the photographing state, the rotating cylinder has a mechanism so as to rotate relative to the fixed cylinder and so as not to move relatively in the optical axis direction, and thereby the lens frame is moved arbitrarily by an engagement of a cam or a cam groove provided in an inner circumferential surface of the rotating cylinder and a cam follower of the fixed cylinder. There is a lens barrel as described above.
Moreover, a lens barrel has been demanded having further miniaturization and therefore engagements of each part have to be smaller and shallower. However, a trend as such weakens impact resistance, for example, for a fall of the lens barrel and so on, and in order to deal with this trend, improving the impact resistance is an urgent need.
On the other hand, the cam with which a lens frame engages draws an angular continuous curve locus in regard to the optical axis and thereby positions of each lens frame can be arranged in arbitrary positions and various focal lengths can be created. The smaller an inclination of the cam to the vertical direction of the optical axis is, the higher a positional accuracy and a tilt of the lens frame in the optical axis direction can be created without a relationship to a stop accuracy of the rotating cylinder, and thereby it is possible to maintain higher accuracy. However, in a case where the inclination of the cam to the vertical direction of the optical axis is small, an impact force caused by a fall and so on is directly transmitted to the cam follower and the male helicoid of the rotating cylinder via the cam follower. And thereby there is such a problem that either of those parts may be broken.
Regarding the impact caused by the fall as described above, for example, Japanese patent publication No. 2000-292846 discloses that it is possible to reduce a load which a cam follower receives by curving a guiding groove for guiding the cam follower, that is, a linearly-guiding groove for guiding the lens frame in the optical axis direction without rotating the lens frame. However, curving the guiding groove as described above may make a lens rotate depending on a movement position of the lens frame, and if a curved part has only a small angle to the optical axis direction, there is no change so that the cam follower still receives the load.
Additionally, for example, Japanese patent publication No. 2000-292845 discloses that breakage can be prevented by providing a guiding wall which abuts on a cam pin, that is, a cam follower when receiving an impact. However, a structure as described above has not only a great limitation on a layout, but also there is no change in a load that the cam follower receives.
On the other hand, for example, Japanese patent publication No. 2003-337276 discloses that in a range of imaging, a load that a cam follower receives can be dispersed into torque for the rotating cylinder and is reduced, not by making a cam surface on an image plane side facing a cam surface on an object side to be parallel to the object side, but by making an inclination angle to a perpendicular direction of an optical axis to be larger than to the object side. Moreover, for example, Japanese patent publication No. 2003-279827 discloses that it is possible for a rotating cylinder not to rotate, to return to a position before receiving a load when the load is removed, and not to be out of focus, if a lens barrel receives an impact from outside, by making at least a part of a cam surface on an image plane side facing a cam surface on an object side not to be parallel to the object side but to be a flat surface which is perpendicular to an optical axis direction.